Researchers at the University of Tokyo have moved a step closer to displays and simple computers that you can wear on your sleeve or wrap around your couch. And they have opened up the possibility of printing such devices, which would make them cheap.
Takao Someya, an electrical-engineering professor, and his colleagues make a stretchable display by connecting organic light-emitting diodes (OLEDs) and organic transistors with a new rubbery conductor. The researchers can spread the display over a curved surface without affecting performance. The display can also be folded in half or crumpled up without incurring any damage.
In a previous Science paper, the researchers used their elastic conductor–a mix of carbon nanotubes and rubber–to make a stretchy electronic circuit. The new version of the conductor, described online in Nature Materials, is significantly more conductive and can stretch to more than twice its original size. What’s more, it can be printed. Combined with printable transistors and OLEDs, this could pave the way for rolling out large, cheap, wearable displays and electronics.
Bendy, flexible electronics that can be rolled up like paper are already available. But rubber-like stretchable electronics offer the additional advantage that they can cover complex three-dimensional objects. “With a sheet of paper, you can wrap a cylinder or a cone, but that’s pretty much it,” says John Rogers, a professor of materials science and engineering at the University of Illinois at Urbana-Champaign. “You can’t wrap a body part, a sphere, or an airplane wing.”
To make such materials, researchers have tried several approaches. Rogers uses ultrathin silicon sheets to make complex circuits on stretchy surfaces–he recently demonstrated a spherical camera sensor using the circuits. Others have made elastic conductors using graphene sheets or by combining gold and rubbery polymers.
The new carbon nanotube conductor offers the advantage of being printable. “The main advance is that they’re able to print elastic conductors that are highly conductive and highly stretchable,” says Stephanie Lacour, who studies stretchable electronic skin at the University of Cambridge, in England. “Printing is cheap, and it allows you to cover large-area substrate.”
Someya first combines carbon nanotubes with an ionic liquid–a liquid containing charged molecules–and a liquid polymer to make a nanotube-rubber paste. Then comes the crucial part: a high-pressure jet that spreads the nanotubes in the rubber.
The jet makes the nanotube bundles thinner without shortening them and disperses the bundles uniformly in the polymer. “The longer and finer bundles of nanotubes can form well-developed conducting networks in rubbers, thus significantly improved conductivity and stretchability,” Someya says. The jet process also increases the material’s viscosity, making it suitable for high-definition screen printing.
The researchers use a printing mask to deposit 100-micrometer-wide lines of the conductor on a piece of rubber. Then they use the lines as a wire grid to connect organic transistors and OLEDs–a transistor addresses each OLED pixel–to make a display that can stretch by up to 50 percent of its original shape. “This work is very impressive,” Rogers says. “The data shows that they can stretch and deform these displays without changing the property of the pixels too much.”
Many other applications could be possible with the stretchable wiring. The researchers could use it to make sensitive artificial skin for robots or prosthetic limbs. Instead of using OLEDs, they would use pressure sensors on the printed conductor. The electrodes could also be used in implantable medical devices to study or repair body organs.
Someya says that Tokyo-based Dai Nippon Printing is interested in commercializing the stretchable display, and a product should be possible in five years. But first, the researchers need to make higher-resolution displays by printing conductor lines narrower than 100 micrometers.